1. Field of the Invention
This invention relates to a thick-film resistor paste containing a glass powder and a conductive material powder, and more particularly to such a thick-film resistor paste suitable for fabricating thick-film resistors on a ceramic substrate.
2. Description of the Prior Art
A thick-film resistor paste of RuO.sub.2 system is conventionally printed on a surface of a ceramic substrate into a thick-film resistor pattern when resistors are fabricated on the substrate surface by a thick-film process. The thick-film resistor pattern is then fired at a temperature ranging between 600 and 900.degree. C. to be fabricated into thick-film resistors. Since a resistance value of the thick-film resistor generally varies after the firing, the thick-film resistor is trimmed by a laser trimming process after the firing so that the resistance value is adjusted.
In the laser trimming, thermal strain sometimes results in occurrence of microcracks in the thick-film resistor. The microcracks gradually progress in the actual use of the ceramic substrate, whereupon the resistance value of the thick-film resistor accordingly varies with time. The variation of the resistance value lowers the reliability of the circuit. The microcracks tend to progress under the condition where the thick-film resistor is subjected to a tensile stress. Accordingly, a compressive force is desired to be applied to the thick-film resistor so that the progress of microcracks is prevented. For this purpose, the thick-film resistor needs to have a smaller thermal expansion coefficient than the ceramic substrate.
However, the thick-film resistor paste conventionally used to fabricate the thick-film resistor on the ceramic substrate consists of a mixture of a glass powder, RuO.sub.2 powder and an organic vehicle. RuO.sub.2 has a thermal expansion coefficient of 6.0.times.10.sup.-6 /deg. On the other hand, a low-temperature fired ceramic substrate has a thermal expansion coefficient of 5.5.times.10.sup.-6 /deg, which value is smaller than a thermal expansion coefficient of an alumina substrate. Accordingly, the glass of the thick-film resistor paste is desired to have a thermal expansion coefficient smaller than 5.5.times.10.sup.-6 /deg. However, a selection range of glass is limited when electric characteristics, mechanical strength, harmfulness, etc. are taken into consideration. Thus, the thermal expansion coefficient of the conventionally used glass is unavoidably approximately equal to or larger than that of the ceramic substrate. In this state of art, the thermal expansion coefficient of the thick-film resistor containing the above-described glass and RuO.sub.2 is approximately equal to that of the ceramic substrate at the smallest. Accordingly, the thick-film resistor is subjected to an insufficient compressive force due the difference between the thermal expansion coefficients of the thick-film resistor and the ceramic substrate. Moreover, a tensile force is sometimes applied to the thick-film resistor. Consequently, the microcracks produced in the thick-film resistor during the laser trimming tends to progress in the actual use, resulting in large variations in the resistance value of the thick-film resistor with progress of time.